1. Field of the Invention
The present invention relates to a method and machine for use in hydrostatic extrusion and more particularly to a method and machine for use in hydrostatic extrusion wherein a billet is placed in a container in sealing relation and the pressure medium is compressed in the container such that the billet is extruded under a triaxial compressive force.
2. DESCRIPTION OF THE PRIOR ART
Although hydrostatic extrusion has many advantages, such as low extruding load, good lubrication and feasibility of extruding a long billet, it has suffered from the disadvantages inherent thereto, in that the cycle time of the extrusion is longer than that of the desired cycles of extrusion which can meet the demand from the production field, since excessive time and man power are required for charging and discharging the pressure medium into and from the container.
Furthermore, with warm or hot hydrostatic extrusion, a billet preheated outside the machine is charged into the container and then the pressure medium is introduced into the container, resulting in the drawback that it takes a considerable length of time before starting the extrusion with an accompanied undersirable tendency of an appreciable degree of decrease in the temperature of the billet due to heat-radiation. A further problem experienced is that, since the pressure medium is introduced into the container after charging the billet into the container, the pressure medium has to be subjected to disturbance, thereby accelerating the heat exchange between the billet and the pressure medium, with the result that thermal efficiency is lowered and undesirable thermal loss is encountered.
Included by the pressure medium for use in such a conventional hydrostatic extrusion is, in genreal, a liquid-state pressure medium such as castor oil. On the other hand, there has been proposed a special method wherein glass is used as a lubricant for hot extrusion in an attempt to enhance the efficacy of the hydrostatic extrusion by using a layer of glass having an increased thickness.
More particularly, this method involves the steps of placing a glass pad molded to a solid state in the front or rear of a billet charged in a container, whereby the glass in contact with the preheated billet may be melted to fill the gap between the billet and the container, and then extrusion follows. In this respect, the glass used serves the dual purposes of a lubricant and a pressure medium. However, this method presents only a partial solution to the aforesaid disadvantages. In other words, although the use of a solid state pressure medium may result in decreasing the time required for charging said medium into the container, as compared with the case of using a conventional liquid state pressure medium, it is impossible to start the extrusion under hydrostatic pressure, immediately after charging the billet into the container, since such a step should be followed wherein the pressure medium in a solid state receives the heat from the billet and thus is melted to fill the gap around the circumference of the billet. This apparently fails to shorten the total time required for extrusion with the accompanied decreased thermal efficiency due to a heat loss for melting the glass. In addition to this, this method can be applied only to the temperature range above the melting point of the glass, and thus the temperature range used is to some extent subjected to limitation.